The invention relates to a device for sensing liquid level in a tank and having signal-generating means for remote indication and/or monitoring of instantaneous liquid level. More particularly, the invention relates to such a device wherein mechanical simplicity, electrical simplicity, safety and reliability are significant factors.
U.S. Pat. Nos. 4,724,706, 4,838,083, and 4,970,896 disclose liquid-level sensing devices which are vertically elongate to define a straight vertically guided path of movement for an annular float which continuously assumes the level of liquid, such as gasoline, in a tank. Float-guidance structure is adapted for suspension from the top of the tank, and instantaneous liquid level is displayed and externally viewable at the top of the device, as an indicator needle which swings in a horizontal plane, against an inscribed scale. The needle is the horizontally bent upper arm of an elongate vertical ribbon along which the float can slide; the vertical ribbon is twisted along its length, to the extent of a given fraction of a full turn, and the float is so keyed to the ribbon that the ribbon and, therefore, also the indicator needle are angularly displaced in accordance with the instantaneous elevation of the float. There is no suggestion of or provision for remote transmission of instantaneous liquid-level data, which must be locally viewed in the devices of these patents.
For remote transmission of liquid-level data, other quite different arrangements exist on the market, and the "STW" sender device of Isspro, Inc., of Portland, Oreg., is illustrative. The Isspro device comprises a free-flooding elongate tube for vertical installation within a tank, there being suitable upper-flange formations at the upper end of the tube for suspension of the same from the top of the tank. An elongate printed-circuit board within the tube mounts a vertically arrayed succession of magnetic-reed switches and resistors at discrete vertical intervals, and a float guided within the tube carries a magnet to actuate successive switches in response to changes of float position. The resistors and switches are so interconnected that an output signal indicative of liquid level is available at a remote location via flexible cabling connected to the upper end of the tube. When the liquid in the tank is a fuel, there is an important question of safety, in that the switches are circuit elements that must operate in fully immersed condition and in fuel-rich vapor above the instantaneous liquid level. Admittedly, these switches are individually encapsulated, but their susceptibility to damage poses the safety question.